FIG. 1 illustrates a cross-sectional view of a conventional solenoidal magnet arrangement for a nuclear magnetic resonance (NMR) or magnetic resonance imaging (MRI) system. A number of coils of superconducting wire are wound onto a former 1. The resulting assembly is housed inside a cryogen vessel 2 which is at least partly filled with a liquid cryogen 2a at its boiling point. The coils are thereby held at a temperature below their critical point.
Also illustrated in FIG. 1 are an outer vacuum container 4 and thermal shield 3. As is well known, these serve to thermally isolate the cryogen vessel 2, typically containing a liquid cryogen 2a, from the surrounding atmosphere. Solid insulation 5 may be placed inside the space between the outer vacuum container 4 and the thermal shield 3. A central bore 4a is provided, of a certain dimension to allow access for a patient or other subject to be imaged.
Conventionally, a number of supporting elements 7 are connected between the cryogen vessel 2 and the outer vacuum container 4 to bear the weight of the cryogen vessel. These may be tensile bands, tensile rods, straps, compression struts or any known element suitable for the purpose. The elements should have a very low thermal conductivity. This is important in order to minimise heat influx from the outer vacuum container 4, which is typically at ambient temperature, to the cryogen vessel 2. The suspension elements typically pass through holes in the thermal shield 3. Similar, or alternative, suspension arrangements may be provided to retain the thermal shield 3.
Copending United Kingdom patent application GB2426545 describes a particularly advantageous arrangement of support straps for use in such systems.
Typically, a floor mounted system will require mounting feet 6 or similar, attached to the outer vacuum container to support the weight of the system as a whole.
A disadvantage of the conventional arrangement results from the fact that the outer vacuum container 4 must carry all of the forces required to support the cryogen vessel 2 and its contents. The outer vacuum container 4 is the largest component in the illustrated system, and must be made of thick material in order to support such heavy loading.
The present invention aims to provide an alternative support arrangement, wherein the outer vacuum container need not be of thick material, since it does not bear the forces required to support the cryogen vessel, or other supported equipment.                According to the present invention, therefore, suspension of the cryogen vessel is provided by tensile bands, tensile rods, straps, compression struts or any known element suitable for the purpose. In particular, these elements are connected between the cryogen vessel 2 and a housing provided on the outer surface of the outer vacuum container.        